Test pieces embodied from staple slivers impregnated with martrices are used in many ways for traction tests. Up until now, traction tests are carried out by traction machines using clamping jaws controlled, for example, by a pneumatic, hydraulic or self-clamping system on test piece slivers without heels whose production proves to be long and costly. In fact, as regards resin matrices, these slivers are manufactured in a machine impregnated with resin from a coil of dry fiber made, for example, of carbon, glass or silicon. The percentage of impregnation of the sliver may be adjusted by a pulley system and the sliver is then wound onto a mandrel and polymerized at 60.degree. C. in an oven. Then, after the sliver is polymerized, carbon or metal heels intended to prevent the sliver from sliding are conventionally glued on the test piece. The production cost of these test pieces is obviously high, and embodying the heels alone is relatively expensive. Apart from this drawback, so as to test the heels, the glue on the heels must first be dried, which may delay the test by 24 hours. On the other hand, this technique no longer allows the test to continue until the new carbon fibers with improved mechanical characteristics are ruptured.
This is why orientation is moving towards the embodiment of test piece sliver without heels, which reduces their cost price and allows time to be saved on conducting tests.
However, with test piece slivers without heels, new difficulties arise due to the fact that sliding of the sliver in the clamping jaw needs be avoided during traction test, in particular, as regards the considerable stresses attained with the use of carbon slivers.
A certain number of techniques are used to prevent the slivers from sliding. For example, self-clamping conical and ribbed jaws are used on currently-used traction machines. If clamping has the effect of increasing the forces on the test piece during traction, it does not eliminate concentration of the force at the jaw outlet. Apart from other test pieces, this is suitable for metallic test pieces. Secondly, they act on the surface of the test piece and create clamping stresses not compatible with test piece slivers without heels. There also exists the technique consisting of coating the clamping jaw with abrasive papers to avoid sliding. But it has been observed that this would involve rapid wear of the abrasive paper and a large consumption of slivers due to the fact that frequently the grain of the paper is too aggressive for the filaments of slivers and causes them to rupture in the jaw. It thus follows that, in order to obtain a value with a small standard difference, it is necessary to break more slivers two or three times during the tests.
Finally, there currently exists a technique consisting of coating the clamping jaws with elastomers. This has the advantage of protecting the sliver when the gripping jaws are reclosed along the sliver. Thus, when the jaws close under the force of the closing pressure, the elastomer imprisons the test piece and thus prevents sliding. Nevertheless, the level of force remains insufficient when this involves testing highly resistant slivers or slivers with a large section. In this case, there is sliding in the jaws or shearing ruptures the elastomer or, when the clamping pressure is increased, the sliver breaks when flush with the jaw. In fact, the traction force in the sliver moves by shearing stresses to the interface between the jaw and the sliver whose field is considerably non-linear. Thus, it is possible to increase the length of the jaws or their clamping force, although the problem would nevertheless be resolved with ruptures still occurring at the outlet of the jaws. In order to avoid this happening, it has been considered using non-conical clamping jaws nesting inside each other, these jaws extending over a certain height so as to be able to take up the traction forces by means of shearing of the soft material mounted on the jaws. But in this case, the operation for placing and removal of the sliver proves to be long and difficult; in addition, the change of internal coatings of the jaws proves to be delicate and requires that the tests be halted for several days. In short, it can be seen that rupture flush with the jaws results in several stresses being superimposed, namely a traction stress distributed in the sliver, a compression stress introduced by the jaws and a shearing stress resulting from the force introduction mode.